1. Field of the Invention
The present invention relates to imaging techniques for creating composite images. More particularly, the present invention relates to color key composite images, sometimes referred to as "blue screen " images, and to surfaces and stages for creating such images.
2. The Prior Art
Color key composite imaging techniques, often referred to as "blue screen" imaging techniques are known in the prior art for creating composite images. A subject is photographed in front of an evenly lit, bright, pure color (usually blue) background. The compositing process, whether photographic or electronic, replaces all the background color in the scene with another image, sometimes referred to as the background plate.
Blue screen composite images can be produced optically, electronically, and digitally. Until very recently all blue screen compositing for films have been done optically and all television composites have been done using analog real-time circuits.
Another term for Blue Screen is Chroma-Key. Chroma-Key is a television process only. The Chroma Key process is based on the Luminance key in which everything in the image having a brightness greater or lesser than a set brightness level is "keyed" out and is replaced by either another image, or a color from a color generator. Luminance key techniques are often used to create titles. A title card having white printing on a black background is prepared and placed in front of a camera. The camera signal is provided to a foreground input on the Luminance keyer. The background video is also fed into the keyer. The keyer is adjusted to cause all the black on the title card to be replaced by the background video over which the white letters appear.
Luminance keying has proved to be satisfactory for displaying titles in composite images, but has not proved to be satisfactory for producing live action composites because people and other live action subjects comprise a wide range of color tones. Certain portions of the live action subject may be very dark, while other portions of live action subjects can approach 100% white, which tends to provide false keying.
Chroma Key creates keys on just one color channel. Broadcast cameras use three independent sensors, one for each color, Red (R), Green (G), and Blue (B). Most modem television cameras output RGB signals separately from a Composite video signal. The original chroma key was probably created by feeding the blue (B) output of a television camera into a keyer. Dedicated chromakeyers are now available which accept all three RGB colors, plus the background composite signal and the foreground composite signal. It is thus now possible to select any color for the key and fine tune the selection of the color. As keyers have become more sophisticated, provision has been made for finer control of the transition between background and foreground. The resulting visual effect is more subtle than that of earlier keyers.
Red, green and blue channels have all been used, but blue has been favored for several reasons. Blue is complementary to flesh tone colors, and since the most common color in most scenes is flesh tone, the opposite color avoids most conflicts. Historically, cameras and film have been most sensitive to blue light, although this is less true today. Sometimes (usually) the background color reflects onto the foreground talent creating a slight blue tinge around the edges. This artifact of the composite process is known as blue spill.
Usually only one camera is used as the Chroma Key camera. This creates a 15 problem on three camera sets; the other cameras can see the blue screen. The screen must be integrated into the set design, and it is easier to design around a bright sky blue than an intense green or red.
A more sophisticated television process is Ultimatte, also the name of the company that manufactures Ultimatte equipment. Ultimatte has been supplying 20 equipment to the video industry for many years. With the recent advent of digital image manipulation using computer graphics technology, Ultimatte compositing techniques are becoming more common in film production too. An Ultimatte unit makes it possible to create composites that include smoke, transparent objects, different shades of blue, and shadows. Ultimatte also markets software that works with other programs to create digital mattes, called Cinefusion.
Lighting design for conventional blue screen images is a complicated matter. There are a number of different philosophies employed in blue screen lighting design. The blue screen must be lit evenly, but foreground subjects may be lit as dramatically as is desired. There is interplay between lighting the foreground and lighting the background.
Backlighting has been one popular technique used to minimize blue spill around the foreground subject. Straw, yellow, or CTO gels have been employed on the lights to help wash out blue spilling on the subject's shoulders and hair. A scene in which the subject does not need to be near the blue backing is easier to light because distance may 15 be put between the subject and the background. Generally it is desired to have the level of light on the backing to be the same as the level on the subject from the key light. In video terms, this is between 60-75 IRE on a waveform monitor, although slightly lower levels will usually work. It is most important for the screen to be evenly lit. If the foreground subject is standing or sitting on blue, then it is quite difficult to provide separate lighting. With primitive chromakey systems, shadows can create a lot of difficulty, and so less artistic flat lighting schemes are used on the foreground subject to minimize the shadows.
Numerous solutions have been proposed to minimize the blue spin artifact resulting from the lighting of the blue screen background. According to one prior-art solution, a dedicated blue screen stage comprises a vertical screen formed from a translucent material lit from behind by blue light. The stage floor is covered by MirrorPlex (a plexiglass mirror product) panels creating a mirrored floor. The floor in effect becomes an extension of the blue vertical wall. This dedicated stage is quite cumbersome; it's often too small for the needed shot and the floor is fragile. In addition, the mirrored floor does not allow for realistic shadows to be cast by a subject. Because shadows are almost always required for realism, this presents a significant drawback.
There are other shortcomings in the prior-art blue screen compositing processes. A major recommendation in blue screen compositing is to keep the subject as far from the blue wall as is practical. However, when a full body shot of a subject is required, blue spill on the subject from the floor is a major problem. In addition, by the very nature of the angle at which it reflects lights in relation to a blue wall, a blue floor makes it difficult to get an even exposure from both surfaces. As a consequence, Directors of Photography tend to compromise the composition by avoiding full body shots.
With the traditional flat blue floor, the flexibility of lighting angles is restricted. For example, directing lights straight down creates bright white spectral reflection (glare) from the point of view of the camera, thus, ruining the solid field of color. Similar unwanted spectral reflection can occur when the artistic needs of a forward-falling shadow is desired. Currently, spectral reflection (glare) is mitigated by polarizing filters. However, these polarizing filter add-ons to the camera lens tend to degrade image quality. Additionally, polarizing filters require an exposure adjustment to the camera and often necessitates time consuming lighting changes.
It is therefore an object of the present invention to provide a blue screen technique which overcomes the shortcomings of the prior art.